Extreme-ultraviolet lithography (EUVL) is currently regarded as a candidate “next generation lithography” (NGL) that offers prospects of substantially finer pattern resolution than currently obtainable using conventional “optical” lithography (i.e., lithography performed using deep-ultraviolet wavelengths of light). These expectations of increased resolution from EUVL stem largely from the fact that, whereas current optical lithography is performed using a wavelength in the range of 150-250 nm, EUVL is performed using a wavelength in the range of 11-15 nm, which is at least ten times shorter than the conventional “optical” wavelengths. Generally, the shorter the wavelength of light used for pattern imaging in microlithography, the finer the obtainable resolution.
In view of the extremely small pattern elements (currently less than 100 nm) that can be resolved using microlithography, including EUVL, the accuracy and precision with which pattern transfer is performed lithographically must be extremely high to ensure proper placement and registration of multiple pattern layers on a substrate and to ensure that the pattern elements are transferred to the substrate with high fidelity. To obtain such high accuracy and precision, extreme measures are taken to control and remove extraneous causes of performance degradation. For example, with current expectations being demanded of microlithography systems to produce pattern features of less than 100 nm, eliminating significant particulate contamination has become paramount.
EUV light is highly attenuated by the atmosphere, and no currently known materials are adequately transmissive and refractive to EUV light for use as EUV lenses. Consequently, EUVL must be performed under high vacuum using reflective optics (mirrors) for illumination of the reticle and for projection of the illuminated pattern from the reticle to the substrate. Even the reticle is reflective rather than being a transmissive reticle as used in conventional optical microlithography.
In optical microlithography the reticle during use typically is protected by a pellicle from particulate contamination. (The pellicle is a transmissive thin film on a frame that covers the patterned surface of the reticle to prevent deposition of particles on the reticle surface.) A pellicle cannot be used with a reticle for EUVL because, in view of the lack of EUV-transmissive materials, the pellicle would absorb and thus block the EUV beam incident to the reticle, leaving substantially no EUV light for projecting the pattern image to the substrate. Thus, the EUVL reticle must be used naked, which leaves the reticle vulnerable to particulate contamination during use. In optical lithography in which the reticle is protected by a pellicle, a particle deposited on the pellicle is sufficiently displaced from the plane of the reticle (i.e., outside the depth of focus) to be unresolved (or at most poorly resolved) on the wafer. A particle on a naked EUVL reticle, on the other hand, is in the plane of the reticle pattern and hence is resolved on the wafer where it likely will flaw the projected pattern image. Hence, for EUVL reticles, particle protection as effective as a pellicle is required.
In EUVL systems currently under development, the reticle is used facing downward, which is helpful in preventing deposition of particles on the reticle surface. However, merely facing the reticle downward is insufficient for keeping the reticle completely clean, and various techniques currently are being developed that are aimed at preventing deposition of any particulate contamination on the reticle without having to use a pellicle. One technique that exhibited remarkable success in preventing particulate deposition on the reticle (by preventing particles from hitting the reticle) is termed “thermophoresis,” discussed in Rader et al., “Verification Studies of Thermophoretic Protection for EUV Masks,” Proceedings SPIE 4688:182-193, 2002. See also U.S. Pat. Nos. 6,153,044 and 6,253,464. Thermophoresis refers to a force exerted on particles in a gas where a temperature gradient is present, wherein the particles are driven by a thermophoretic “force” (imparted by the gas) from a warmer region to a cooler region. Thus, a surface can be protected from particle deposition by maintaining the surface at a warmer temperature than its surroundings.
General principles of thermophoresis as applied in an EUVL system are described with reference to FIG. 8, which depicts a reticle 222 and a nearby surface 226 that is maintained at a cooler temperature than the reticle 222. The cooler surface 226 may be, for example, a differential pumping barrier used in a vacuum chamber housing the reticle 222 or a shield that protects the reticle. A gas in the vicinity of the reticle 222 and the surface 226 exhibits a temperature gradient in which the gas is warmer near the reticle 222 and cooler near the surface 226. The thermophoretic “force” associated with the gradient urges particles 228 away from the warmer reticle 222 toward the cooler surface 226. Some particles 228 may actually become attached to the surface 226. Thermophoretic forces are greatest in the presence of a sufficient gas pressure in which the mean free path of the gas molecules is a small fraction of the distance from the reticle 222 and the surface 226. As pressure is decreased (i.e., as vacuum is increased), thermophoretic forces decrease correspondingly. In other words, thermophoresis is poorly effective in high vacuum, but at a pressure of 50 mTorr thermophoresis is still significant for effectively urging particles 228 away from the reticle 222.
A conventional thermophoretic scheme as disclosed in the references cited above is generally shown in FIG. 9, which depicts a portion of an EUVL system 100 in the vicinity of the reticle. The depicted system 100 comprises a vacuum chamber 104 including a first region 108 and a second region 110. The first region 108 contains a reticle stage 114 that supports a reticle chuck 118 configured to hold a reticle 122 facedown. The second region 110 contains projection optics 124 and a wafer stage (not shown). The first and second regions 108, 110 are substantially separated from each other by a barrier wall 126 through which an opening 130 is defined. The barrier wall 126 and opening 130 collectively form a differential pumping barrier. The opening 130 is sufficiently large to pass EUV light incident to and reflected from the reticle 122. Gas at a pressure of approximately 50 mTorr is supplied to the first region 108 via a gas-supply port 132 in the vacuum chamber 104. To minimize EUV-absorption losses to ambient gas, the second region 110 is maintained at a lower pressure (i.e., higher vacuum; e.g., ≦1 mTorr) than the first region 108. Maintaining these two respective pressures in the regions 108, 110 is achieved by differential evacuation of the regions, performed using respective vacuum pumps 134, 136 and facilitated by the differential pumping barrier.
In the configuration shown in FIG. 9, to remove particles away from the reticle 122 by thermophoresis, the reticle is maintained at a higher temperature than the barrier wall 126. This temperature differential, as discussed above, results in attraction of the particles to the barrier wall 126, which causes some particles (entrained in gas passing through the opening 130) to enter the second region 110 via the opening 130. The flow of gas from the region 108 to the region 110 also helps convey particles away from the reticle 122 and thus prevents the particles from contacting the reticle.
While placing a cooler surface proximal to a warmer reticle helps reduce particulate contamination of the reticle, maintaining surfaces of different temperatures within the EUVL system can be problematic. For example, maintaining surfaces at different temperatures can complicate temperature control of critical subsystems and can generate issues relating to thermal expansion and distortion of critical components. For example, thermal expansion or distortion of the reticle can compromise the performance of the overall EUVL lithography process and hence of the semiconductor-device-fabrication process. Also, flowing gas from the region 108 to the region 110 may sweep particles originating in the region 108 toward the reticle 122, which would increase the risk of contamination despite the general protection afforded by thermophoresis.
Other manners of solving this problem are described elsewhere by the current Applicant, namely U.S. patent application Ser. No. 10/898,475, entitled “Extreme Ultraviolet Reticle Protection Using Gas Flow Thermophoresis,” filed on Jul. 23, 2004, and a corresponding PCT CIP Application, entitled “Extreme Ultraviolet Reticle Protection,” filed on Jul. 23, 2005, both of which being incorporated herein by reference. Briefly, a space is defined between the reticle and a nearby surface, such as a barrier wall or reticle shield. At least one gas nozzle is situated in the space. A gas, cooled to below the temperature of the reticle and surface (the reticle and surface normally have substantially the same temperature), is discharged from the nozzle(s) into the space. The discharged gas, flowing substantially parallel to the reticle, establishes local temperature gradients adjacent the reticle and surface, respectively. The temperature gradients engender respective thermophoretic forces tending to urge particles away from the reticle and surface so that the particles become or remain entrained in the gas.
A particular configuration of the apparatus 300 described in the '475 application is shown in FIG. 10, which depicts a reticle 302 supported by a reticle chuck 304 mounted face down on a reticle stage 306. The reticle stage 306, reticle chuck 304, and reticle 302 are contained in a reticle chamber 308 that is separated from a projection-optics chamber 310 by a barrier wall 312 (e.g., a differential pumping barrier or reticle shield). The barrier wall 312 defines an aperture 314 that is sized and configured to allow illumination EUV light 316 to impinge on the desired region of the reticle 302 and to pass patterned EUV light 318 reflected from the reticle to the projection optics (not shown). The aperture 314 also helps establish and maintain the differential pressures in the two chambers 308, 310 (the reticle chamber 308 is typically at approximately 50 mTorr, and the projection-optics chamber 310 is typically at less than 1 mTorr). During exposure, to illuminate successive regions of the reticle 302, the reticle stage 306 moves in a scanning manner relative to the aperture 314. Flanking the aperture 314 and extending upward (in the figure) toward the reticle 302 is a nozzle manifold 320a, 320b that defines nozzle openings 322a, 322b for discharging the gas. The nozzle openings 322a, 322b are oriented so as to discharge the gas into the space 324 (between the reticle 302 and the barrier wall 312) in a direction substantially parallel to the reticle. The flow of gas (note arrows 326) past the reticle 302 is approximately laminar.
As noted above, the gas can be cooled before discharging the gas into the space 324 between the reticle 302 and barrier wall 312. Alternatively, the nozzle openings 322a, 322b are sized and configured to establish a substantially higher gas pressure at the nozzle openings than in the space 324. Thus, discharge of the gas is accompanied by adiabatic cooling of the gas. I.e., as the gas is discharged into the space 324, it expands rapidly out of the nozzle openings 322a, 322b and cools significantly in the process. With such a configuration, the discharged gas is colder than the reticle 302 and barrier wall 312 and establishes the desired temperature gradient without having to pre-cool the gas. In addition, the relatively high gas pressure at the nozzle openings 322a, 322b produces a high gas-flow velocity through the space 324. This high-velocity flow establishes a substantial viscous-drag force on particles and tends to convey the particles out of the space 324 and thus away from the reticle 302.
As indicated by the multiple arrows 326, most of the discharged gas (and entrained particles) flows laterally as shown, substantially parallel to the reticle 302, through the space 324 and is exhausted via the vacuum pump (not shown but see item 134 in FIG. 9) that evacuates the reticle chamber 308.
Referring further to FIG. 10, the nozzle manifold 320a, 320b extends upward (in the figure) and forms respective narrow gaps G between the “tops” of the nozzle manifold and the surface of the reticle 302. These gaps G, each approximately 1 mm or less, allow limited movement of the reticle 302 (in the vertical, or “z,” direction) as required for focus control and reticle-wafer alignment movements. The narrow gaps G also allow a limited flow of gas (note single arrows 330 compared to multiple arrows 326) from the space 324 through the aperture 314 to the projection-optics chamber 310. The gas flow through the gaps G is limited so as to maintain the desired vacuum level in the projection-optics chamber 310 for minimal attenuation of the EUV illumination and patterned beams.
In the scheme summarized above the pressure in the gaps G is substantially lower than, for example, the pressure in the space 324 between the reticle 302 and the barrier wall 312. This reduced pressure produces a correspondingly reduced thermophoretic “force” in the gaps G compared to elsewhere on the reticle 302. Consequently, particles urged into these gaps by the higher-velocity gas flow near the nozzle openings 322a, 322b have opportunities to contact the reticle 302 and become attached to it.
Therefore, a need exists for methods and devices for reducing this source of particulate contamination of the reticle.